JP 2013-42647A discloses a plurality of secondary battery cells. The plurality of secondary battery cells are installed in a vehicle. Also, the plurality of secondary battery cells are connected in series to each other, are charged by a generator, for example, and supply power to the vehicle load. The generator generates power and outputs a direct voltage. In JP 2013-42647A, a first switch is provided between a high-potential terminal of each of the secondary battery cells and a high-potential terminal of the generator, and a second switch is provided between a low-potential terminal of each of the secondary battery cells and a low-potential terminal of the generator.
In JP 2013-42647A, the secondary battery cells are individually charged by controlling these first and second switches using the generator. For example, a first secondary battery cell is charged by turning on the first switch and the second switch that are connected to the first secondary battery cell. Next, these first and second switches are turned off, and the first switch and the second switch that are connected to a second secondary battery cell are turned on. Accordingly, the second secondary battery cell is charged. Similarly, the secondary battery cells can be individually and evenly charged by sequentially controlling the first switches and the second switches. Note that Patent Documents 2 and 3 are also listed here as other techniques.
An increase in the storage capacity of a power storage device is desired accompanying an increase in the energy consumption of the vehicle load. In view of this, it is conceivable to increase the capacity of the power storage device by connecting cells in parallel to each other. For example, a plurality of cell groups may be adopted. The cell groups each include a plurality of cells that are connected in parallel to each other. Moreover, these plurality of cell groups are connected in series to each other. Accordingly, the capacitor of the power storage device can be increased.
Incidentally, the resistance value of an internal resistance of the cell increases due to aging. In the cell groups including such cells (also referred to as “deteriorated cell group” hereinafter), the charge current decreases. Thus, such cell groups are not easily charged compared to the other cell groups. Therefore, the voltage of any deteriorated cell group may be lower than the voltage of the other cell groups immediately after the charging has ended. In this case, electric current flows from the other cell groups to the deteriorated cell group.
Also, the discharge current flowing from the deteriorated cell group having a large internal resistance to the vehicle load is smaller than the discharge current of the other cell groups having a smaller internal resistance. Thus, the voltage of the deteriorated cell group may be larger than the voltage of the other cell groups immediately after discharge has ended. In this case, electric current flows from the deteriorated cell group to the other cell groups.
If such an electric current flows between the cell groups, this electric current causes losses due to the internal resistance of the cell groups. Thus, such electric current generation is undesirable. That is, charge movement between a plurality of cell groups is undesirable.
In view of this, an object of the present invention is to provide a power supply device for a vehicle in which charge movement between a plurality of cell groups can be suppressed.